The invention generally relates to rotary cone rock bits. More particularly, the invention relates to rotary cone bits having nozzle arrangements to provide improved cutting structure cleaning.
Roller cone bits, variously referred to as rock bits or drill bits, are used in earth drilling applications. Typically, these are used in petroleum or mining operations where the cost of drilling is significantly affected by the rate that the drill bits penetrate the various types of subterranean formations. There is a continual effort to optimize the design of drill bits to more rapidly drill specific formations so as to reduce these drilling costs.
Rotary cone rock bits attach to sections of drilling pipe that connect together to form a drill string. A rock bit attaches to the end of this drill string and is rotated, drilling a bore hole into the earth. Rock fragments known as drill cuttings are generated at the bottom of the borehole by the cutting and grinding action of the drill bit rotating at the bottom of the bore hole. These rock fragments are carried uphole to the surface by a moving column of drilling fluid that travels to the interior of the drill bit through the center of an attached drill string, and is ejected from the face of the drill bit through a series of jet nozzles, and is carried uphole through an annulus formed by the outside of the drill string and the borehole wall. The drilling fluid also maintains borehole integrity, and cleans and cools the face of the rock bit.
A drill bit is configured with a number of roller cones, typically three, at its bottom that are equidistantly spaced around the circumference of the bit. A layout of a three-cone rock bit is shown in FIG. 1A. A rock bit 10 typically comprises a steel bit body and three cutter cones mounted on legs 14 extending from the bottom of the body. The cones are imbedded with inserts 26 (also described as cutting elements or teeth) that penetrate the formation as the drill bit rotates in the hole. It should is appreciated by those of ordinary skill in the art that there are some inserts or teeth on a roller cone that do not cut because they do not extend adequately from the cone surface. These may be identified by their substantially lower height. The upper end of the body is threaded (not shown) and serves as the pin for assembly of the rock bit onto a drill string for drilling bore holes. The area between the legs on the underside of the body is referred to as the dome 18. It should be noted that in describing the invention, an inventive drill bit may be described as though the central axis of the drill bit is vertical and words like “upper”, “lower”, “top”, “bottom”, “above”, and “below” should be interpreted with this in mind, unless the context clearly indicates otherwise.
A cutter cone is rotatably mounted on each of the legs 14. A conventional internal structure may be used to mount the cutter cones on the legs. Each cutter cone has a hollow, generally conical steel body 20. The cones have an apex or nose 22 on one end and an opening on the other 24 for receipt of a journal. Each leg has a bearing journal (not shown) extending from it. Each cone is fitted over a journal, i.e., the journal is positioned inside the cone through the cone's opening. Ball bearings or other cone retention system (not shown) hold the cones on the journals. When mounted on the journals, the cones are radially oriented about the bit central axis so that the nose of the cones is closer to the axis than the opening of the cones.
Cutting elements 26, such as teeth or inserts, are pressed into holes or machined on the external surface of the cones. Unless noted otherwise, the terms teeth and inserts are used interchangeably herein. Cutting elements 26 provide the drilling action by engaging subterranean rock formation as the rock bit is rotated. The inserts may be any type or shape so long as they cut formation. For longer life, the inserts may be tipped with a super hard material (e.g. polycrystalline diamond).
The cutting elements are typically arranged in annular rows on the cone. Although nomenclature varies across the industry, the row that cuts the largest diameter within the bore of the hole is referred to as the gage row 28. The gage row cuts to about gage, i.e. the full diameter of the drill bit. The term “about” is used in this context because due to design or wear the inserts may cut slightly over or under gage but not to an extent that affects the action of the drill bit. For example, a gage row insert may extend a sixteenth of inch short of the gage diameter under certain situations and still be effective. The next cutting closest row is the row that is referred to as the drive (or off-gage) row 27. This row typically generates the highest torque on the cone. The row closest to the apex of the cone is referred to as the nose row 30. For cones comprising a single central cutting element on their apex, the nose row is the central cutting element.
The drawing of FIG. 1A is schematic in its illustration of the inserts. The inserts are illustrated on each cone in apparently overlapping positions. These inserts represent the inserts on all three of the cones projected around to the planes illustrated. This illustrates the complete coverage of the bore hole bottom by inserts during a complete revolution of the roller cones. In actuality, about ⅓ of the insert rows are on each cutter cone and the insert rows are arranged on the individual cones so that they do not interfere with rows of inserts on the adjacent cones.
The center jet of FIG. 1A can be used to illustrate the general construction of a nozzle. A jet assembly 32 is located in a nozzle receptacle bit body dome 18. In some drill bits, an outer sleeve 71 is welded into the dome of the bit body. The jet assembly preferably has a nozzle 34 with a shoulder that seats on a shoulder 72 in the sleeve and a jet bore axis (not shown). An inner retainer sleeve 66 is threaded into the outer sleeve for securing the jet against the shoulder. An O-ring 74 seals between the outer sleeve and the jet body to prevent washout around the jet body.
Generally, between each pair of cones is a nozzle receptacle with an installed erosion resistant nozzle that directs the fluid from the face of the bit to the hole bottom to move the cuttings from the proximity of the bit and up the annulus to the surface. The placement and directionality of the nozzle receptacles and nozzles, as well as the nozzle sizing and nozzle extension, significantly affect the rate of penetration for the drill bit and bit life.
Referring to FIG. 1B, nozzle positioning for a three cone rock bit is shown. In general, a three cone bit has three symmetrical legs, each with a journal. Each of the legs takes up a 120° section, with each of the three legs in a three cone rock bit being 120° apart. The journals, even though they have journal angle and offset associated with them, are also 120° apart, projected on a plane normal to the bit axis. For a two cone bit, the legs and journals can be apart by 180° or 165° or any other degree. The angular difference between the journal axes (at the leg centers) can be termed as the “phase angle” or phase angle difference. Journal axes 102, 104, 106 are each offset radially from bit central axis 108 by a distance defined by a circle 110. By virtue of even spacing around the drill bit, each of the three journal axes are spaced 120 degrees from its neighbors. Three nozzle receptacle locations are shown: nozzle-1 112, nozzle-2 114, and nozzle-3 116. Each nozzle receptacle 112, 114, 116 is located midway between the adjacent journal axes 102, 104, 106. This results in a 120 degree spacing between the nozzle receptacles.
The amount of energy available at the bit is generally dictated by factors external to the bit such as the drilling rigs' available hydraulic energy, drill pipe type, bottom hole assembly (BHA) configuration and drill depth. However, once the available energy for the rock bit is determined, properly configuring the hydraulics of the bit for the specific application can significantly affect the rate of penetration (ROP) of the bit in the formation.
The optimal placement, directionality and sizing of each nozzle can change depending on the bit size and formation type that is being drilled. For instance, at the very soft end of the formation spectrum there is a strong tendency for clay minerals to adhere to the teeth or inserts of bits. The adhesion of formation to teeth or inserts is commonly referred to as “bit balling”. As is known in the art, bit balling describes the packing of formation between the cones and bit body, or between the bit cutting elements, while cutting formation. When it occurs, the cutting elements are packed off so much that they don't penetrate into the formation effectively, tending to slow the rate of penetration for the drill bit (ROP). For example, “gumbo” in the US Gulf Coast area has a sticky nature and adheres to rock bit cutting structures. It must be removed efficiently to maintain reasonable penetration rates.
In harder clays and shales, cuttings can become impacted or “balled up” between the teeth or inserts of the cutting structures. When formation sticks to cones or is impacted between cutting elements it limits insert/tooth penetration. Also, formation packed against the cone-shell closes the flow channels needed to carry other cuttings away. This promotes premature bit wear. In either instance, fluid directed toward the cones can help to clean the inserts and cones, allowing them to penetrate to a greater depth, maintaining the rate of penetration for the bit. Furthermore, as the inserts begin to wear down, the bit can drill longer since the cleaned inserts will continue to penetrate the formation even in their reduced state.
In part to combat bit balling, and in part to allow for larger cones, the cutting elements on different rolling cones often are designed to intermesh. Intermeshing reduces bit balling. As a cutting element of one cone intermeshes between the cutting elements of another cone, it dislodges balling between the cutting elements. Having the cutting elements intermesh also allows the diameter of the cones to be larger, providing for a larger bearing surface which results in a more durable cone. One aspect of intermeshing is different spacing between various rows of inserts on the cones. Of particular interest, the spacing of the gage row and drive row vary amongst each of the roller cones.
FIG. 2A is a bottom view of a hypothetical three cone rock bit (not to scale). First nozzle receptacle 102, second nozzle receptacle 104, and third nozzle receptacle 106 are located between three roller cones 108, 110, and 112. Fluid columns 114, 116, 118 can be seen projecting from respective nozzle receptacles. Although it is to be understood that the jet of fluid ejected from each nozzle receptacle behaves in a complicated manner, to simplify understanding of the invention each fluid discharge is depicted as a column to emphasize its direction. It should also be noted that a revolved surface can be seen around each row of cutting elements rather than the individual cutting elements themselves.
FIGS. 2B–2E (not to scale) show the relative positions of the fluid columns to the roller cones. FIG. 2B shows nozzle fluid stream 118 in proximity to cone 112. Also shown is gage row 202 and drive row 204. FIG. 2C shows nozzle fluid stream 116 in proximity to cone 110. Also shown is gage row 206 and drive row 208. FIG. 2D shows nozzle fluid stream 114 in proximity to cone 108. Also shown is gage row 210 and drive row 212. FIG. 2E shows a composite view of FIGS. 2B–2D. It can be seen that the three fluid streams, 114, 116, 118, appear identically located when superimposed with respect to each fluid stream's respective roller cone. Each fluid stream is about the same distance from the gage rows (or more precisely, the surface of revolution for the gage row inserts) 202, 206, 210 on the respective roller cone. It can be seen that the spacings from the inserts on other rows 204, 208, 212 on the roller cones to the corresponding fluid stream varies significantly from cone to cone, however.
Numerous efforts have been made by drill bit designers to solve the problem of bit balling yet the problem persists. Known drill bit designs vary placements, directionality and sizing of nozzles and fluid streams in an attempt to maximize performance and rate of penetration of drill bits in various formations. However, the type, angle placement and specifics of nozzle receptacles for any given bit are the same. This may be due to the manner in which drill bits are typically manufactured. Typically, a drill bit is assembled from three identical portions. These portions are welded together to form the drill bit body. The roller cones are then attached to the bottom of the bit body. Further, perhaps to avoid installation problems at the drilling site, the nozzles installed in each nozzle receptacle are typically identical. However, while the relative location of the nozzle receptacles and nozzles may therefore be the same, the roller cones on the bit bottom are not. The spacing of the rows of inserts on each roller cone differs from the spacing of the rows of inserts on its neighboring cones because of intermeshing. As a result, in a three cone rock bit, the conventional hydraulic configuration either cleans one cone very well and compromises cleaning of the others, or a less-than-optimum configuration exists to clean cutting structures on all three cones.
Bit balling remains a problem. Ideally, a drill bit could be designed that reduces the effects of bit balling while drilling the borehole. In at least some embodiments, this drill bit would not require complicated installation of varying, differently sized, or differently configured nozzles at the drill site.